Current mode logic latch

ABSTRACT

A current mode logic latch may include a sample stage and a hold stage, the hold stage comprising first and second stage transistors, first and second hold stage current sources, and a hold stage switch. The first hold stage transistor may be coupled at its drain terminal to the drain terminal of a first sample stage transistor. 
     The second hold stage transistor may be coupled at its drain terminal to the drain terminal of a second sample stage transistor, coupled at its gate terminal to the drain terminal of the first hold stage transistor, and coupled at its drain terminal to a gate terminal of the first hold stage transistor. The first hold stage current source may be coupled to a source terminal of the first hold stage transistor. The second hold stage current source may be coupled to a source terminal of the second hold stage transistor. The hold stage switch may be coupled between the source terminal of the first hold stage transistor and the source terminal of the second hold stage transistor.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of copending U.S.patent application Ser. No. 13/495,786 filed Jun. 13, 2012, which ishereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

The present invention relates generally to electrical circuits, and moreparticularly, to current mode logic latches.

BACKGROUND

High-speed latches and flip-flops are useful in many applications (e.g.,data sampling, clock deskew, etc.) in the electrical arts and are oftenan essential part of design of digital circuits. Various differentstructures and architectures have been proposed for high-speed latch andflip-flop designs, including static, dynamic, single phase, multiphase,clocked complementary metal-oxide-semiconductor (CMOS), transmissiongate, and many others. Increasingly, circuit designers are employingcurrent mode logic (CML) latches in their designs due to their superiorperformance at very high frequencies and other advantages as compared toother latch architectures.

An example of a traditional CML latch 100, as known in the art, isdepicted in FIG. 1. A current mode logic latch typically comprises asample stage 102, a hold stage 104, and a current source 106. Samplestage 102 may comprise transistors 108 a, 108 b, and 110, and drainresistors 112 a and 112 b. Each drain resistor 112 a, 112 b may becoupled between the drain terminal of a corresponding transistor 108 a,108 b, respectively, and a high potential voltage source (e.g., V_(DD)).Each transistor 108 a, 108 b may in turn be coupled at its gate terminalto a differential input (e.g., V_(in) ⁺ or V_(in) ⁻) and at its sourceterminal to the drain terminal of transistor 110. Transistor 110 may becoupled at its gate terminal to a periodic clock signal CLK and coupledat its source terminal to current source 106.

Hold stage 104 may comprise transistors 114 a, 114 b, and 116. Each oftransistors 114 a, 114 b may be coupled at its drain terminal to thedrain terminal of a corresponding transistor 108 a, 108 b, respectively,and at its source terminal to the drain terminal of transistor 116. Inaddition, transistors 114 a and 114 b may be cross-coupled such that thegate terminal of transistor 114 a is coupled to the drain terminal oftransistor 114 b, and vice versa.

In operation, sample stage transistors 108 a, 108 b form a differentialtransistor pair to sense and track the input data represented by aninput voltage V_(in), and cross-coupled hold stage transistors 114 a,114 b form a differential transistor pair to store the sensed data.These differential pairs are switched by the complementary signals ofthe periodic clock signal CLK and CLK′. When CLK is of a high voltagepotential (and CLK′ is accordingly of a low voltage potential),substantially all of the current sourced by current source 106 passesthrough sample stage 102. On the other hand, when CLK is of a lowvoltage potential (and CLK′ is accordingly of a high voltage potential),substantially all of the current sourced by current source passesthrough hold stage 104. Thus, the periodic clock signal CLK periodicallyenables and disables each of sample stage 102 and hold stage 104 in acomplementary fashion (e.g., sample stage 102 is enabled while holdstage 104 is disabled, and vice versa).

The differential pair of sample stage 102 functions as a CML inverter,driven by complementary data signals. When V_(in) is of a high voltagepotential, substantially all of the current sourced by current source106 passes through resistor 112 a and transistor 108 a and as a result,the voltage at the drain terminal of transistor 108 a may obtain a lowvoltage potential (e.g., V_(DD)−R_(D)I_(ss), where R_(D) is theresistance of each of resistors 112 a and 112 b). At the same time,substantially no current passes through transistor 108 b, and as aresult, the voltage at the drain terminal of transistor 108 b may obtaina high voltage potential (e.g., V_(DD)). Accordingly, CML latch 100produces a differential output signal V_(out) equal to the differencebetween the voltage potential at the drain terminal of transistor 108 b(e.g., the node labeled “y” in FIG. 1) and voltage potential at thedrain terminal of transistor 108 a (e.g., the node labeled “x” in FIG.1). The differential cross-coupled transistor pair of hold stage 104forms a regenerative positive feedback structure which maintains V_(out)during the time in which hold stage 104 is enabled by the complementaryclock signal CLK′. FIG. 5 depicts example waveforms for V_(in), CLK, andV_(out) illustrating functionality of a CML latch.

Despite the advantages of conventional CML latches, in many technologies(e.g., submicron CMOS technologies), conventional CMOS latches maypresent voltage headroom problems. As an example, consider a submicrontechnology in which a typical power supply voltage V_(DD) is 1.0V. Notuncommonly, a power supply may have 5% to 10% tolerance variations.Therefore, with a 1.0V V_(DD), the worst case power supply may be 0.9V.In the circuit of FIG. 1, the output voltage swing is set exclusively bythe amount of tail current (I_(ss)) and the value of the resistor load(R_(D)) and is generally much smaller than V_(DD), in the order of a fewhundred millivolts. Consider further that the CML latch is to be used inan application in which a single-ended-peak output swing of 0.2V isrequired. With a single-ended-peak output swing of 0.2V, the outputcommon mode voltage of the latch circuit is 0.7V (e.g., V_(DD)−0.2V).The conventional CML latch 100 may suffers from severe voltage headroomproblems when designed in a 1.0V (worst case 0.9V) supply voltage. WhenCLK is of a high voltage potential, the common-mode voltage at theinputs (e.g., V_(in) ⁻ and V_(in) ⁻) has to be sufficiently high enoughto ensure that transistors 108 a, 108 b, and 110 and a transistor (notshown) of current source 106 are operated in saturation mode. Similarly,when CLK′ is of a high voltage potential, the common-mode voltage at theoutputs (e.g., V_(out) ⁺ and V_(out) ⁻)needs to drive transistors 114 a,114 b, and 110 and a transistor (not shown) of current source 106 intosaturation mode. Therefore, the common-mode voltage at both the inputsand the outputs need to satisfy the following relation:

V _(DD)−Swing>V _(CM) >V _(TH-NMOS)+2V_(DSAT-NMOS)

where V_(DD) is the supply voltage, Swing is the output voltage swing,W_(CM) is the common mode voltage; V_(TH-NMOS) is the threshold voltageof an NMOS transistor (e.g., transistor 108 a, 108 b, 110, 114 a, 114 b,116 or a transistor of current source 106), which may, by way ofexample, be about 0.4 to 0.5V in a submicron technology; andV_(DSAT-NMOS) is the saturation drain-to-source voltage of the NMOStransistor which may, by way of example, be about around 0.2V in asubmicron technology. As a result, a V_(CM) larger than at least 0.8V(0.4V+2*0.2V) is required to operate the latch circuit properly. With aworst case VDD of 0.9V and a single-ended-peak output swing of 0.2V (andtherefore an output common-mode voltage of 0.7V), the output common-modevoltage may be insufficient to drive the transistors into saturationmode. The design may therefore suffer from severe voltage headroomproblems. The latch circuit cannot acquire a wide input common-moderange, and the output swing of the latch circuit is small reducing therobustness of the circuit. In addition, with the tail current source inthe triode region, the power supply rejection (i.e., rejection of noisefrom the power supply) may be poor resulting in noisy output waveforms.

Addressing these problems in submicron CMOS technologies may bedifficult because the threshold voltage V_(TH-NMOS) and the saturationdrain-to-source voltage V_(DSAT-NMOS) often do not shrinkproportionately with reductions in the power supply. Removing currentsource 106 as depicted in FIG. 2 is one possible method of addressingthe headroom issue, thus relying on transistors 110 and 116 to provide atail current. With this proposed solution, however, the tail currentwould be inaccurate due to at least the uncertainty of the common-modevoltage and the voltage swing of the CLK and CLK′ signals, which mayresults in an inaccurate output common-mode voltage as well as outputswing. Also, with an inaccurate tail current, the transconductance ofsample stage transistors 108 a and 108 b may vary significantly,requiring oversized transistors 108 a and 108 b for proper trackingoperation. Another possible solution to address the headroom issue is touse low threshold voltage transistors. However, this requires extra masksteps in the manufacturing process that may not be available in atypical CMOS process. The extra mask steps to lower the thresholdvoltage also lay also render manufacturing costs prohibitivelyexpensive.

SUMMARY

In accordance with some embodiments of the present disclosure, a currentmode logic latch may include a sample stage and a hold stage. The samplestage may include a first sample stage transistor, a second sample stagetransistor, a first resistor, a second resistor, a first sample stagecurrent source, a second sample stage current source, and a sample stageswitch. The first sample stage transistor may be coupled at its gateterminal to a first input terminal of the current mode logic latch. Thesecond sample stage transistor may be coupled at its gate terminal to asecond input terminal of the current mode logic latch, the first inputterminal and the second input terminal comprising a differential voltageinput. The first resistor may be coupled between a drain terminal of thefirst sample stage transistor and a high potential voltage source. Thesecond resistor may be coupled between a drain terminal of the secondsample stage transistor and the high potential voltage source. The firstsample stage current source may be coupled to a source terminal of thefirst sample stage transistor. The second sample stage current sourcemay be coupled to a source terminal of the second sample stagetransistor. The sample stage switch may be coupled between the sourceterminal of the first sample stage transistor and the source terminal ofthe second sample stage transistor. The hold stage may comprise a firsthold stage transistor, a second hold stage transistor, a first holdstage current source, a second hold stage current source, and a holdstage switch. The first hold stage transistor may be coupled at itsdrain terminal to the drain terminal of the first sample stagetransistor. The second hold stage transistor may be coupled at its drainterminal to the drain terminal of the second sample stage transistor,coupled at its gate terminal to the drain terminal of the first holdstage transistor, and coupled at its drain terminal to a gate terminalof the first hold stage transistor. The first hold stage current sourcemay be coupled to a source terminal of the first hold stage transistor.The second hold stage current source may be coupled to a source terminalof the second hold stage transistor. The hold stage switch coupledbetween the source terminal of the first hold stage transistor and thesource terminal of the second hold stage transistor.

Technical advantages of the present disclosure may be readily apparentto one skilled in the art from the figures, description and claimsincluded herein. The objects and advantages of the embodiments will berealized and achieved at least by the elements, features, andcombinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 illustrates a schematic diagram of an example current mode logic(CML) latch, as is known in the art;

FIG. 2 illustrates a schematic diagram of another example CML latch, asis known in the art;

FIG. 3 illustrates a schematic diagram of an example CML latch employinga two-phase clock, in accordance with certain embodiments of the presentdisclosure;

FIG. 4 illustrates a schematic diagram of another improved example CMLlatch employing a single-phase clock, in accordance with certainembodiments of the present disclosure; and

FIG. 5 illustrates example waveforms depicting example functionality ofboth traditional CML latches and improved CML latches disclosed herein.

DETAILED DESCRIPTION

The present disclosure may refer to the “size” of various types oftransistors, including an n-type metal-oxide semiconductor field-effecttransistor (NMOS). Unless otherwise specified, the description of atransistor's size, as used herein, describes the size parameter thataffects the transconductance of the transistor. For example, for NMOSdevices, “size” may refer to the width-to-length ratio of the gateand/or of the conducting channel of the device. Accordingly, a devicethat is described as being sized at a ratio as compared to anotherdevice may have a transconductance that is larger or smaller at thatratio as compared to the transconductance of the other device.

Further, the term “equivalent” may be used to describe two or morecurrents or two or more voltage potentials that may be designed to beapproximately equal to each other. Though they may be designed to beapproximately equal to each other, “equivalent” voltages, “equivalent”currents, or other “equivalent” items may include some variation due tofactors including, but not limited to, device matching imperfections,semiconductor processing imperfections, and/or imbalanced operatingconditions.

FIG. 3 illustrates a schematic diagram of an example CML latch 300employing a two-phase clock, in accordance with certain embodiments ofthe present disclosure. Power may be supplied to CML latch 300 through ahigh potential power supply and a low potential power supply. For thepurposes of this disclosure, a high potential power supply may bereferred to as “V_(DD)”, and a low potential power supply may bereferred to as “GND” or “V_(SS)”.

As shown in FIG. 3, CML latch 300 may comprise a sample stage 302 and ahold stage 304. Sample stage 302 may comprise transistors 308 a, 308 b,a switch 310, resistors 312 a, 312 b, and current sources 306 a, 306 b.Each resistor 312 a, 312 b may be coupled between the drain terminal ofa corresponding transistor 308 a, 308 b, respectively, and a highpotential voltage source (e.g., V_(DD)). Transistors 308 a, 308 b may inturn be respectively coupled at their gate terminals to oppositepolarities of a differential input (e.g., V_(in) ⁺ or V_(in) ⁻). Inaddition, each transistor 308 a, 308 b may be coupled at its sourceterminal to a corresponding current source 306 a, 306 b, respectively.In some embodiments, transistors 308 a and 308 b may be of approximatelythe same size. Switch 310 may be coupled between the source terminals oftransistors 308 a and 308 b and may be enabled and disabled by aperiodic clock signal (e.g., CLK).

Hold stage 304 may comprise transistors 314 a, 314 b, switch 316, andcurrent sources 307 a, 307 b. Each of transistors 314 a, 314 b may becoupled at its drain terminal to the drain terminal of a correspondingsample stage transistor 308 a, 308 b, respectively. Furthermore, eachtransistor 314 a, 314 b may be coupled at its source terminal to acorresponding current source 307 a, 307 b, respectively. In addition,transistors 314 a and 314 b may be cross-coupled such that the gateterminal of transistor 314 a is coupled to the drain terminal oftransistor 314 b, and vice versa. In some embodiments, transistors 314 aand 314 b may be of approximately the same size. Switch 316 may becoupled between the source terminals of transistors 314 a and 314 b andmay be enabled and disabled by a periodic clock signal complementary tothat enabling/disabling switch 310 (e.g., CLK′).

In operation, sample stage transistors 308 a, 308 b form a differentialtransistor pair to sense and track the input data represented by aninput voltage V_(in), and cross-coupled hold stage transistors 314 a,314 b form a differential transistor pair to store the sensed data.These differential pairs are switched by the complementary signals ofthe periodic clock signal CLK and CLK′, such that the periodic clocksignal CLK periodically enables and disables each of sample stage 302and hold stage 304 in a complementary fashion (e.g., sample stage 302 isenabled while hold stage 304 is disabled, and vice versa).

The differential pair of sample stage 302 functions as a CML buffer,driven by complementary data signals. Such complementary data signalsmay themselves be CML signals. To illustrate operation of CML latch 300,consider that for a positive value of V_(in), transistor 308 a willexperience a gate voltage of V_(DD) (e.g., 0.9V) and transistor 308 bwill experience a gate voltage of V_(DD)−V_(swing) (e.g.,0.9V−0.3V=0.6V), where V_(swing) is the voltage swing at V_(in). WhenV_(in) is positive and sample stage 302 is enabled (e.g., CLK is high),switch 310 is closed and current sources 306 a and 306 b are combined.The voltage at the drain terminal of transistor 308 a (denoted as node xis FIG. 3), may obtain a voltage V_(x)=V_(DD)−R_(D)I_(SS) _(—)₃₀₆−I_(SS) _(—) _(307b)R_(D), where I_(SS) _(—) ₃₀₆ is the sum of thecurrents of current sources 306 a and 306 b (I_(SS) _(—) ₃₀₆=I_(SS) _(—)_(306a)+I_(SS) _(—) _(306b)). In addition, the voltage of drain terminalof transistor 308 b (denoted as node y in FIG. 3), may obtain a voltageVy=V_(DD)−I_(SS) _(—) _(307a)R_(D). As a specific example, considerR_(D)=150Ω, I_(SS 306a)=I_(SS 306b)=I_(SS 307a)=I_(SS 307b)=0.5 mA. Insuch a case, with V_(in) positive and sample stage 302 enabled,V_(x)=0.675V and V_(y)=0.825V. If, on the other hand, V_(in) isnegative, V_(x)=0.825V and V_(y)=0.675V.

When CLK is low and hold stage 304 is enabled, hold stage 304 will holdthe value at nodes x and y regardless of the value at V_(in). Continuingthe numerical example from above, assume CLK goes from high to low whenV_(x)=0.75V and V_(y)=0.6V. At such time, switch 316 may close, andcurrent sources 307 a and 307 b are effectively combined. Because V_(x)is a higher potential than V_(y), transistor 314 b will source morecurrent than transistor 314 a. Assuming that transistor 314 b sources1.75 mA of the total 2 mA of current of transistors 307 a and 307 b,V_(x)=V_(DD)−(I_(SS) _(—) ₃₀₆+0.25mA)R_(D)=0.785V andV_(y)=V_(DD)−(I_(SS) _(—) ₃₀₆+1.75 mA)R_(D)=0.562V. Thus, for thisparticular numerical example, the sampled signal having a voltage swingof 0.15V on the output of CML latch 300 during the sample stage may havean output voltage swing of 0.785V−0.562V=0.223V during the hold stage.Thus, depending on device sizes, current levels, and other parameterschosen for design, the swing levels in the sample stage and the holdstage may vary.

Although FIG. 3 depicts that sample stage 302 is enabled while clocksignal CLK is of a high voltage potential and hold stage 304 is enabledwhile clock signal CLK is of a low voltage potential, other embodimentsin which sample stage 302 is enabled while clock signal CLK is of a lowvoltage potential and hold stage 304 is enabled while clock signal CLKis of a high voltage potential are contemplated herein.

In addition, although FIG. 3 depicts current sources 306 a, 306 b, 307a, and 307 b all generating tail currents equal to I_(SS), otherembodiments in which currents generated by current sources 306 a and 306b may be substantially different than those generated by current sources307 a and 307 b.

Simulations confirm that CML latch 300 possesses operationalcharacteristics substantially similar to that of traditional CML latches(e.g., CML latch 300 performs substantially in accordance with thewaveforms depicted in FIG. 5). However, the topology of CML latch 300,when compared to traditional CML latches, effectively removes onetransistor between the path between VDD and GND, thus potentiallyimproving voltage headroom while ensuring tail current sources (currentsources 306 a, 306 b, 307 a, and 307 b) remain in saturation.

FIG. 4 illustrates a schematic diagram of an example CML latch 400employing a single-phase clock, in accordance with certain embodimentsof the present disclosure. Power may be supplied CML latch 400 through ahigh potential power supply and a low potential power supply. As shownin FIG. 4, CML latch 400 may comprise a sample stage 402 and a holdstage 404. Sample stage 402 may comprise transistors 408 a, 408 b,resistors 412 a, 412 b, and current sources 406 a, 406 b. Each resistor412 a, 412 b may be coupled between the drain terminal of acorresponding transistor 408 a, 408 b, respectively, and a highpotential voltage source (e.g., V_(DD)). Transistors 408 a, 408 b may inturn be respectively coupled at their gate terminals to oppositepolarities of a differential input (e.g., V_(in) ⁺ or V_(in) ⁻). Inaddition, each transistor 408 a, 408 b may be coupled at its sourceterminal to a corresponding current source 406 a, 406 b, respectively.Furthermore, transistors 408 a, 408 b may be coupled to each other attheir source terminals. In some embodiments, transistors 408 a and 408 bmay be of approximately the same size. In some embodiments, currentsources 406 a and 406 b may be combined into a single current source.

Hold stage 404 may comprise transistors 414 a, 414 b, switch 416,resistor 418 and current sources 407 a, 407 b. Each of transistors 414a, 414 b may be coupled at its drain terminal to the drain terminal of acorresponding sample stage transistor 408 a, 408 b, respectively.Furthermore, each transistor 414 a, 414 b may be coupled at its sourceterminal to a corresponding current source 407 a, 407 b, respectively.In addition, transistors 414 a and 414 b may be cross-coupled such thatthe gate terminal of transistor 414 a is coupled to the drain terminalof transistor 414 b, and vice versa. In some embodiments, transistors414 a and 414 b may be of approximately the same size. Switch 416 may becoupled between the source terminals of transistors 414 a and 414 b andmay be enabled and disabled by a periodic clock signal (e.g., CLK orCLK′). Resistor 418 may have a resistance R_(s) and may be coupledbetween the source terminals of transistors 414 a and 414 b.

In operation, sample stage transistors 408 a, 408 b form a differentialtransistor pair to sense and track the input data represented by aninput voltage V_(in), and cross-coupled hold stage transistors 414 a,414 b form a differential transistor pair to store the sensed data. Thedifferential pair of sample stage 402 functions as a CML inverter,driven by complementary data signals.

Periodic enabling and disabling of switch 416 by a periodic clock signal(e.g. CLK or CLK′) may have the effect of periodically changing theeffective load resistance seen at the output terminals of CML latch 400.If the transconductances of transistors 408 a, 408 b, 412 a, and 412 b(and thus, the overall transconductance of CML latch 400) and/orresistance R_(S) of resistor 418 is appropriately tuned, the effectiveload resistance may be configured such that it has a positive effectiveresistance when switch 416 is disabled and a negative effectiveresistance when switch 416 is enabled. Thus, in a sample mode in switch416 is disabled, the positive effective resistance may leads toamplification of an input signal. On the other hand, in a hold mode inwhich switch 416 is enabled, the negative resistance may lead topositive feedback at the output and permit components of hold stage 404to outdrive components of sample stage 402, thus permitting hold stage404 to maintain a sampled signal at the output of CML latch 400.

To further illustrate, those of skill in the art may appreciate that theeffective load resistance R_(L) as seen at the output V_(out) of CMLlatch 400 may be given by:

R _(L) =R _(D)/(1−G _(m) ·R _(D))

where R_(D) is the resistance of resistors 412 a, 412 b and G_(m) is theeffective transconductance as seen at the output V_(out) of CML latch400. The transconductance may be given by:

G _(m) =g _(m2)/(1+g _(m2) ·R _(S)/2); when switch 416 is disabled; and

G_(m)=g_(m2); when switch 416 is enabled; and

where g_(m2) is the transconductance of each transistor 414 a and 414 b.Thus, by selecting appropriate values of g_(m2), R_(D), and/or R_(S),CML latch 400 can be configured such that the output load resistance ispositive when switch 416 is disabled and negative when switch 416 isenabled.

Although FIG. 4 depicts that switch 416 is enabled while clock signalCLK is of a low voltage potential and vice versa, other embodiments inwhich switch 416 is enabled while clock signal CLK is of a high voltagepotential are contemplated herein.

In addition, although FIG. 4 depicts current sources 406 a, 406 b, 407a, and 407 b all generating tail currents equal to I_(SS), otherembodiments in which currents generated by current sources 406 a and 406b may be substantially different than those generated by current sources407 a and 407 b.

Simulations confirm that CML latch 400 possesses operationalcharacteristics substantially similar to that of traditional CML latches(e.g., CML latch 400 performs substantially in accordance with thewaveforms depicted in FIG. 5). However, the topology of CML latch 400,when compared to traditional CML latches, effectively removes onetransistor between the path between VDD and GND, thus potentiallyimproving voltage headroom while ensuring tail current sources (currentsources 406 a, 406 b, 407 a, and 407 b) remain in saturation. Inaddition, use of a single-phase clock may eliminate clock skew that mayoccur in the case of a latch using a differential clock.

As used herein, a current source (e.g., current source 306 a, 306 b, 307a, 307 b, 406 a, 406 b, 407 a, and/or 407 d) may include any electricalor electronic device configured to deliver or absorb electric current.

As used herein, a switch (e.g., switch 310, 316, and/or 416) maycomprise any system, device, or apparatus configured to selectivelybreak or close an electrical path based on a received control signal,thus disabling or enabling the flow of current through the switch. Aswitch is considered to be enabled when it is selected to enable flow ofcurrent, and considered to be disabled when it is selected to disableflow of current. A switch may be implemented in any suitable fashion,including, without limitation, as a transistor or a transmission gatecomprising a plurality of transistors. As used herein, a transistor(e.g., transistor 308 a, 308 b, 314 a, 314 b, 408 a, 408 b, 414 a,and/or 414 b) may comprise any system, device, or apparatus configuredto control a signal at a pair of its terminals by a signal applied toanother pair of its terminals. Although the FIGURES of this disclosuredepicts transistors as n-type complementary metal-oxide-semiconductorfield-effect transistors, transistors may include any other suitabletype of transistor (e.g., n-type complementary metal-oxide-semiconductorfield-effect transistors, bipolar junction transistors, junction-gatefield effect transistors, insulated gate bipolar transistors, etc.).

As used herein, a resistor (e.g., resistor 312 a, 312 b, 412 a, 412 b,418) may comprise any electronic component that produces a voltageacross its terminals that is proportional to the electric currentpassing through it in accordance with Ohm's law.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding the invention andthe concepts contributed by the inventor to furthering the art, and areconstrued as being without limitation to such specifically recitedexamples and conditions. Although embodiments of the present inventionshave been described in detail, it should be understood that variouschanges, substitutions, and alterations could me made hereto withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A latch, comprising: a sample stage having adifferential input and a differential output; and a hold stage coupledto the differential output of sample stage and configured to provide aneffective resistance at the differential output that is positive duringa first phase of the latch and negative during a second phase of thelatch.
 2. The latch of claim 1, wherein the hold stage comprises: a holdcircuit configured to have a transconductance that is larger during thesecond phase than during the first phase; and a current sourceconfigured to bias the hold circuit during the first phase and duringthe second phase.
 3. The latch of claim 1, wherein the sample stagecomprises: a sample circuit configured to have a consistenttransconductance across the first phase and the second phase; and asample-stage current source configured to bias the sample circuit duringthe first phase and during the second phase.
 4. A latch, comprising: asample stage having a differential input and a differential output; anda hold stage configured to alternate between a first phase and a secondphase of the latch, the hold stage comprising: a first current sourceand a second current source; a hold-stage resistor coupled between thefirst current source and the second current source; a hold-stage switchcoupled between the first current source and the second current source,the hold-stage switch configured to be open during the first phase andto be closed during the second phase; and a cross-coupled pair oftransistors coupled to the differential output of the sample stage andconfigured to be biased by the first current source and the secondcurrent source during the first phase and during the second phase. 5.The latch of claim 4, wherein: the sample stage further comprises samplecircuit configured to have a consistent transconductance across thefirst phase and the second phase; and the latch further comprises: afirst load device coupled to a first differential output node of thesample stage; and a second load device coupled to a second differentialoutput node of the sample stage.
 6. The latch of claim 4, wherein atransconductance of the hold stage is larger during the second phasethan during the first phase.
 7. The latch of claim 4, wherein, duringthe first phase, a transconductance of the sample stage is larger than atransconductance of the hold stage.
 8. The latch of claim 4, wherein,during the second phase, a transconductance of the hold stage is largerthan a transconductance of the sample stage.
 9. The latch of claim 4,wherein the cross-coupled pair of transistors aremetal-oxide-semiconductor (MOS) transistors.
 10. The latch of claim 9,wherein: the cross-coupled pair of transistors comprises a firsttransistor and a second transistor; a drain of the second transistor anda gate of the first transistor are coupled to a first load device at afirst differential output node of the sample stage; and a drain of thefirst transistor and a gate of the second transistor are coupled to asecond load device at a second differential output node of the samplestage.
 11. The latch of claim 10, wherein the first load device is afirst load resistor and the second load device is a second loadresistor.
 12. A current mode logic latch, comprising: a sample stagecomprising: a first sample stage transistor coupled at its gate terminalto a first input terminal of the current mode logic latch; a secondsample stage transistor coupled at its gate terminal to a second inputterminal of the current mode logic latch and coupled at its sourceterminal to a source terminal of the first sample stage transistor, thefirst input terminal and the second input terminal comprising adifferential voltage input; a first sample stage resistor coupledbetween a drain terminal of the first sample stage transistor and a highpotential voltage source; a second sample stage resistor coupled betweena drain terminal of the second sample stage transistor and the highpotential voltage source; and at least one sample stage current sourcecoupled to the source terminal of the first sample stage transistor andthe source terminal of the second sample stage transistor; and a holdstage comprising: a first hold stage transistor coupled at its drainterminal to the drain terminal of the first sample stage transistor; asecond hold stage transistor coupled at its drain terminal to the drainterminal of the second sample stage transistor, coupled at its gateterminal to the drain terminal of the first hold stage transistor, andcoupled at its drain terminal to a gate terminal of the first hold stagetransistor; a first hold stage current source coupled to a sourceterminal of the first hold stage transistor; a second hold stage currentsource coupled to a source terminal of the second hold stage transistor;a hold stage switch coupled between the source terminal of the firsthold stage transistor and the source terminal of the second hold stagetransistor; and a hold stage resistor coupled between the sourceterminal of the first hold stage transistor and the source terminal ofthe second hold stage transistor.
 13. The current mode logic latch ofclaim 12, wherein the hold stage switch is enabled during a first phaseof a periodic clock signal and disabled during a second phase of theperiodic clock signal complementary to the first phase.
 14. The currentmode logic latch of claim 13, wherein at least one of the resistance ofthe first sample stage resistor, the resistance of the second samplestage resistor, the resistance of the hold stage resistor, thetransconductance of the first hold stage transistor, and thetransconductance of the second hold stage transistor is selected suchthat an effective resistance between the drain terminal of the firstsample stage transistor and the drain terminal of the second samplestage transistor is negative during the first phase and positive duringthe second phase.
 15. The current mode logic latch of claim 12, whereinthe first sample stage resistor and the second sample stage resistorhave approximately the same resistance.
 16. The current mode logic latchof claim 12, wherein the first hold stage current source and the secondhold stage current source are configured to generate approximately thesame current.
 17. The current mode logic latch of claim 12, wherein theat least one sample stage current source, the first hold stage currentsource, and the second hold stage current source are configured suchthat the at least one sample stage current source generate approximatelythe same current as the combination of the first hold stage currentsource and the second hold stage current source.
 18. The current modelogic latch of claim 12, further comprising: the drain terminal of thefirst sample stage transistor coupled to a first output terminal of thecurrent mode logic latch; and the drain terminal of the second samplestage transistor coupled to a second output terminal of the current modelogic latch, the first output terminal and the second output terminalcomprising a differential voltage output.
 19. The current mode logiclatch of claim 12, wherein at least one of the first sample stagetransistor, the second sample stage transistor, the first hold stagetransistor, and the second hold stage transistor comprises ametal-oxide-semiconductor transistor.
 20. The current mode logic latchof claim 12, wherein at least one of the first sample stage transistor,the second sample stage transistor, the first hold stage transistor, andthe second hold stage transistor comprises an n-typemetal-oxide-semiconductor transistor.